MR perfusion methods are used to measure blood flow in different body regions such as cerebral blood flow (CBF) in a patient head. One set of MR methods uses injection of contrast agents like Gadolinium in the blood to measure the wash-in and wash-out behaviour in imaged slices of patient anatomy such as in the brain. Another set of MR methods does not require a contrast agent, but uses magnetic tagging of blood, e.g., by applying magnetically inverted spins in blood in the neck for use as an “intrinsic contrast agent” referred to as arterial spin labelling (ASL). In ASL, blood with the tagged spins perfuses into an anatomical region of interest (ROI), where it is measured with fast MR imaging methods. In ASL, images are most often acquired with a perfusion-sensitive preparation (tag) image and non-perfusion-sensitive (control) image. For quantification of CBF, multiple experimental and physiological parameters have to be properly calibrated and taken into account. Pulsed ASL (PASL) methods use a short inversion pulse for tagging. For example, a 10 cm inversion slice is placed in the neck region and image slices are acquired from parts of the brain tissue.
Inversion time (TI) is the delay time between an MR inverting pulse and a slice read out time as indicated by an RF excitation pulse (and corresponding RF echo pulse). At present, perfusion quantification is used to account for different TISLICE (by applying an inverted factor ΔMCORRECTED=ΔM.exp(ΔTI/T1′), which normalizes difference in TISLICE for different slices relative to a first slice designated TISLICE—1. This is valid and gives substantially correct CBF values, if a bolus (tagged blood or contrast agent) arrives in time in regions of a slice that is imaged and is fully collected. This model is valid for a maximum transit time Δt up to (TISLICE—1−τ)<0.8 at a first slice. However, pathological retardation and slower circulation in brain regions may make bolus transit times larger than the maximum transit time and the model calculation underestimates the true CBF in these regions. It is known that a global value of CBF is reduced and increased transit times occur in elderly humans or in stroke patients where transit times may exceed 1 second. It has been shown in multiple TI times measured for the same slice in a healthy human brain that in some regions the model and quantification fit well, but in other regions considerable bolus dispersion and latency are observed.
A known system for addressing this problem involves acquisition of multiple inversion times (TI) but, for example, 10 inversion time points increase experiment imaging time 10-fold to approx. 36 min (compared to a typical four minute PASL scan). Further, such a known system involves the definition of brain regions in ROI data which requires a first manual or automated brain segmentation and the fitting of different kinetic models to ROI data (standard model, Hrabe model, extended model incorporating tag bolus width). A single voxel model fit is too noisy. Known systems addressing the problem of unknown prolonged or dispersed bolus transit time in healthy and pathological conditions require considerable experimental and computational effort and imaging time. Measurement times that are much longer than 5-10 minutes are often not feasible for clinical applications due to limited time available and due to the potential accumulation of artefacts (e.g., resulting from patient motion during scans). A system according to invention principles addresses these deficiencies and associated problems.